1. Field of the Invention
The present invention relates to a fusion splicing structure of a rare earth-added double clad fiber and a single clad fiber for light amplification used in optical fiber amplifiers and optical fiber laser light sources, and in particular to a fusion splicing structure which can be increased in service life of a fiber coating material and greatly improved in reliability by managing appropriately the heat dissipation at a fusion splicing point.
2. Description of Related Art
In optical fiber amplifiers and optical fiber laser light sources, a double clad fiber is used as optical fibers for light amplification. The double clad fiber is made up of a core region to which rare earth is added, a first clad region disposed around the core region and a second clad region also disposed around the first clad region. In the double clad fiber, excitation light is propagated through the core region and the first clad region, and signal beams are propagated through the core region. Since rare earth ions are excited by excitation light which is made incident, induced emission light is disseminated from the rare earth ions upon propagation of signal beams inside the core region, thereby amplifying the signal beams. If a single clad fiber is spliced to the output end, excitation light is not propagated but only the signal beams are propagated through the core, thereby outputting high-quality signal beams.
Conventionally, there have been disclosed technologies for fusion splicing structures of optical fibers, for example, as shown in the following Patent Documents 1 to 6.
Patent Document 1 discloses a structure in which a protective coating is not damaged when leaked light from double clad fibers is 1 W. In this conventional technology, a material high in heat resistance is used as a coating material.
Patent Document 2 discloses that a transparent ultraviolet cured resin is used as a fiber coating material to suppress the absorption of leakage light resulting from bending. In this conventional technology, a primary coating layer is greater in refractive index than a clad layer, and a secondary coating layer is greater in refractive index than the primary coating layer.
Patent Document 3 discloses a structure in which a double clad coating is partially removed and which is used to coat a material higher in refractive index than a first clad.
Patent Document 4 discloses treatment in which alumite, gold, or silver etc., is deposited on the inner wall to absorb light.
Patent Document 5 discloses that light is absorbed by black alumite treatment.
Patent Document 6 discloses a structure in which a first and a second reinforcement substrate are fixed by using a soft adhesive agent and both ends of a fiber and a first reinforcement substrate are fixed by an adhesive agent.
Patent Document 1: Japanese Patent No. 3433900
Patent Document 2: PCT International Publication No. WO 2004/66007
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2000-252559
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 10-10353
Patent Document 5: U.S. Pat. No. 6,597,853
Patent Document 6: Japanese Unexamined Patent Application, First Publication No. H5-34540
Since excitation light being propagated through the first clad of the double clad fiber is absorbed by rare earth ions, the excitation light attenuates while propagation. However, substantially 10% of excitation light is not absorbed by rare earth ions but propagated as it is. Therefore, at a part of splicing the double clad fiber and the single clad fiber, excitation light is emitted to a space from a fusion splicing point, some of which is absorbed by a coating material while propagation through a clad region of the single clad fiber, and thereby a coating material and thereby converted into thermal energy. In a case where signal output is amplified to high power, for example, several watts to several hundred watts, the amount of excitation light must be increased to result in an increased heating value of coating materials. Thus, generally used coating materials may be discolored or burnt when used for a prolonged time, which contributes to a decrease in reliability.
Further, in a case where optical fibers are subjected to fusion splicing with each other, a general practice is that the fusion splicing point is reinforced. However, if a resin having a higher refractive index than air is used to reinforce the fusion splicing point, the numerical aperture (hereinafter, abbreviated as NA) is decreased at a fusion splicing point and also high-power excitation light is released from the fusion splicing point. Therefore, there is a possibility that a reinforcement resin in contact with this point may burn, and the reinforcement resin and a coating is not brought closer to the fusion splicing point.
Still further, the loss of signal beams also takes place at a fusion splicing portion, leaked signal beams are propagated through a clad region of the single clad fiber, absorbed by a coating material and converted into thermal energy. A great signal loss may damage a coated portion. Since signal beams are fewer in NA, there is a case where a coating at the side of the proximal end several millimeters to several dozen millimeters from a fusing point may be damaged.
The present invention has been made in view of the above situation, an object of which is to provide a fusion splicing structure in which at a fusion splicing portion of a double clad fiber and a single clad fiber, a fusion splicing point is appropriately managed for heat dissipation, thus making it possible to extend the service life of a fiber coating material and also greatly improve the reliability.